<?php
//isset你用对了吗
$map = array('',' ','0','1','true','false','null',0,1,true,false,null);
foreach ($map as $a){
    echo '值：'.print_r($a,true).PHP_EOL;
    if(isset($a)){
        echo "存在".PHP_EOL;
    }else{
        echo "不存在".PHP_EOL;
    }
    echo "------".PHP_EOL;
}